Low-loss continuously variable delay line



Nov. 23, 1965 H. BRUECKMANN LOW-LOSS CONTINUOUSLY VARIABLE DELAY LINE 2Sheets-Sheet 1 Filed Oct. 9, 1963 INVENTOR,

HELMUT BRUECKMANN.

BY #444 1. 7. J M 2 IO 0',ANGULAR SLOT WIDTH W/ M W ATTORNEE,

Nov. 23, 1965 H. BRUECKMANN LOW-LOSS GONTINUOUSLY VARIABLE DELAY LINE 2Sheets-Sheet 2 Filed Oct. 9, 1963 INVENTOR, HEL MUT BRUECKMAN/V.

ATTORNEE United States Patent f .Helmut Brueckmann, Little Silver, N.J.,assignor to the United States of America as represented by the Secretaryof the Army Filed Oct. 9, 1963, Ser. No. 315,093 11 Claims. (Cl. 33331)(Granted under Title 35, US. Code (1952), sec. 266) The inventiondescribed herein may be manufactured and used by or for the Governmentfor governmental purposes without the payment of any royalty thereon.

This invention relates to delay lines and more particularly to alow-loss, continuously variable, delay line for use in RF frequencies athigh power levels.

One type of variable delay line well known in the art is essentially ahelical transmission line provided with a sliding contact. This type ofvariable delay line is characterized by high losses unless it is made ofthick wire, in which, case it becomes rather heavy and bulky. Also, thesliding contacts are a potential source of trouble, par ticularly inapplications involving high RF power, and the delay does not varycontinuously. Other types of known high-frequency variable delay linesare limited in their use by one of the above mentioned factors.

It is an object of the present invention to provide an improved delayline wherein the aforementioned limitations are overcome.

It is another object of the present invention to provide an improvedcontinuously variable delay line which has no contacts of any sort andpermits the use of large size conductors.

In brief, there is provided a continuously variable, low-loss delay linewhich includes a transmission line section comprising a pair ofconductors and a ferrite dielectric adapted to be slidably positionedrelative to the conductors. The parameter of the ferrite dielectric ischosen such that the characteristic impedance of the transmission lineremains constant regardless of the relative position of the ferritedielectric with respect to the conductors. The ferrite dielectric isslidably positioned with respect to the conductors such that in oneextreme position, the ferrite dielectric is coextensive with theconductors, and in the other extreme position, the ferrite dielectric iscompletely removed from the spacing between the conductors and replacedby an air dielectric. The delay of an RF signal applied to thetransmission line may be continuously varied between a minimum delayproduced when the ferrite dielectric is completely removed, and amaximum delay when the ferrite dielectric is coextensive with theconductors.

For a better understanding of the invention, together with other andfurther objects thereof, reference is had to the following descriptiontaken in connection with the accompanying drawings in which:

FIGS. 1 and 2 are end and side sectional views, respectively, of a delayline according to the teachings of the invention with FIG. 1 being asectional view along the lines 11 of FIG. 2;

FIG. 3 is a graph useful in designing the delay line shown in FIGS. 1and i2; and

FIGS. 4, 5 and 6 illustrate other embodiments of the present invention.

Referring now to FIGS. 1 and 2 of the drawing, there is shown at 10 acoaxial-type transmission line section having an outer conductor 12, aninner conductor 14 and a tubular ferrite dielectric 16. The innerconductor 14 is supported and held fixed at its ends by a pair oflongitudinally spaced, parallel-arranged, radial conductors 18 and 20which extend through and are atfixed to insulator plugs 22 and 24respectively provided therefor. As

3,219,950 PatentedNov. 23, 1965 shown, insulator plugs 22 and 24 areaffixed to outer conductor 12 to form an integral part thereof. Tubularferrite dielectric 16 is provided with a longitudinal slot 26 having awidth slightly larger than the width of radial conductors 18 and 20 sothat the ferrite dielectric 16 may be longitudinally actuated along theaxis of the transmission line. The ferrite dielectric 16 isconcentrically positioned with respect to outer and inner conductors 12and 14 and is spaced therefrom so that an air space exists between theinner longitudinal periphery of tubular ferrite 16 and inner conductor14, and also between the outer longitudinal periphery of ferrite 16 andouter conductor 12. The surfaces bounding longitudinal slot 26 are shownin FIG. 1 as being radial and include the angle 6 therebetween. With theslot 26 aligned with the radial conductors 18 and 20, the ferritedielectric 16 may be slidably positioned along the longitudinal axis ofthe coaxial line section 10 so that the spacing between the inner andouter conductors 14 and 12 includes all of ferrite dielectric 16 when itis in one extreme longitudinal position, and only air separates the twoconductors 12 and 14 when the ferrite dielectric 16 is at its otherextreme longitudinal position. Of course, any suitable means well knownin the art may be used to slidably position ferrite dielectric 16longitudinally intermediate the outer and inner conductor of line 10 asdescribed above. The high-frequency input signals to be delayed areapplied to coaxial section 10 through radially disposed conductor 18 andthe delayed output signal is derived from radially disposed conductor20. As shown, the outer conductor 12 is terminated at the output end ofthe line by a metallic plate 30 which is spaced from the end of innerconductor 14. The spacing is chosen such that the lumped capacity due toend plate 30 and the lump inductance of radial conductor 20 effectivelycomprise a 1r type filter having a characteristic impedance whichmatches that of the coaxial line section 10. Electrically, this isequivalent to a low pass filter circuit having a cutoff frequency abovethe operating sig nal frequency range. The dimensions of the line shownin FIG. 1 are shown to be 2r for the diameter of the inner conductor 14;2R for the inner diameter of ferrite dielectric tube 16; 2R for theouter diameter of ferrite dielectric tube 16; 2R for the inner diameterof outer conductor 1-2; and B the angle included between the radiallyarranged peripheral surfaces bounding slot 26. Although the ferritematerial within the line 10 has been termed a dielectric, it should benoted that this material also has magnetic properties since its relativepermeability is greater than unity; and it is the combination of itsdielectric and magnetic properties which affect the parameters of a linecontaining ferrite. The arcuate or circumferential spacing of the slot26 is so chosen that the charac teristic impedance of the line does notchange regardless of the relative position of the tubular ferritedielectric 16 with respect to the outer and inner conductors 12 and 14.This will occur when the effective relative permeability ,u and theeifective relative dielectric c of line section 10 are equal when theair spacing between the inner and outer conductors is replaced byferrite dielectric 16. However, the propagation velocity relative tolight velocity C is reduced in accordance with the following equationThis means, of course, that a variable delay may be obtained by firsthaving the ferrite dielectric tube 16 coextensive with and intermediatethe outer and inner conductors 12 and 14 of line 10, and then graduallymoving the ferrite tube longitudinally from the line until it iscompletely removed therefrom. The delay can thus be varied continuouslybetween a minimum delay which is produced .point was achieved when 6=27.

in the latter case and a maximum delay which is produced in the formercase when the ferrite dielectric is completely within coaxial linesection 10.

The following mathematical analysis will provide a better understandingof the operation of the invention. Using the dimensions shown in FIGS. 1and 22, it can be shown that r where ,u is the relative permeability ofthe ferrite. This relation shows that gthe relative effectivepermeability is considerably smaller than the relative permeability offerrite dielectric even if the slot is only a small fraction of thecircumference of the conductors 12 and 14. The effective dielectricconstant of the line is also smaller than the relative dielectricconstant e;- of the ferrite dielectric and can be shown to be R 1 R R.21: Zn Zn -Hn It can be seen from Equations 2 and 3 that both he and eare functions of the slot width 6. FIG. 3 shows a plot of am and eggaccording to Equations 2 and 3 as a function of the slot width 6 for acommercially available ferrite known as Q and characterized by a ,u flland an e,=1-1. For reasons of simplicity, the clearance between theferrite'dielectric 16 and the outer and inner conductors 12 and 14 hasbeen neglected. In FIG. 3,'the ratio of the outer and inner radii R/ rof line was arbitrarily chosen so that the characertisti-c impedance ofthe line 10 without the tubular ferrite dielectric 16 is 60 ohms. Asshown in'FIG. 3, ,u Fe at the point the curves cross and, with the otherabove mentioned parameters, this This means, of course, that a slotwidth of about 27 results in a line whose characteristic impedance doesnot change when the tubular ferrite-dielectric body is removed andaccordingly replaced by air dielectric. However, the propagationvelocity V relative to the light velocity C is reduced in accordancewith Equation 1. Thus, for example, a ten nanosecond delay, whichcorresponds to 180 phase shift at 50 mc., requires a ferrite body lengthof only 30 cm. completely within the coaxial line section. The delay isreduced tcr one nanosecond, which corresponds to 18 phase shift at 50mc., when the ferrite body is completely removed from the coaxial linesection. Between the limits of one and 10 nanoseconds, of course, thedelay is continuously variable.

Although the slot width of the line shown in FIG. 1 is angular, that is,bounded by radially disposed surfaces, it need not be so limited. A slotof uniform width for example may be utilized and the relationship of thevalues of he and e to the slot width can-be established empirically ortheoretically. A uniform slot width has the advantage of resultingin amore uniform flux density versus radius in the ferrite and thus increasethe power handling capacity of the 'line ifit is limited by hysteresislosses alone.

FIGS. 4 and 5 illustrate the strip-line and two wire line embodiment,respectively, of the present invention. Like reference numerals refer tolike parts. In FIG. 4, the ferrite dielectric 16 is shown as beingslidably positioned with respect to the conductor 14 which is parallelto ground plane 12 of the strip-line. In FIG. 5, the ferrite dielectric'16 is shown as being slidably positioned with respect to the two wiretransmission line 10. While the provision necessary to hold theconductors in place and yet permit sliding the ferrite dielectric bodyinto and out of the transmission line section may initially cause anexcess of capacitance and inductance at the terminals,

such discontinuities may be compensated for by conventional techniquessuch as appropriate changes in conductor spacing and/or conductor size.

FIG. 6 illustrates an embodiment of the invention wherein two delaylines are combined back-to-back to provide a difierential delay line.Referring now to FIG. 6, there is shown at 40 a coaxial line sectionwhich has two axially spaced aligned and colinear inner conductors 42and 44 and a common outer conductor 46 terminated by end plateconductors 48 and 50. As in FIGS. 1 and 2, the end plates 48 and 50 areappropriately spaced from the ends of inner conductors 44 and 42respectively. Spaced radial conductors 52 and 54 support inner conductor44 within line 40 and similarly spaced radial conductors 56 and 58support inner conductor 42 within line 40.

As in FIGS. 1 and 2, all the radial conductors are linearly aligned. Ametallic cylinder 60 is axially positioned intermediate the innerconductors 42 and 44 inorder to isolate or decouple one line from theother. The tubular ferrite dielectric 62 is disposed intermediate theinner conductors 42 and 44 and is spaced therefrom as described inconnection with FIGS. -1 and 2. Ferrite dielectric 62 is provided with alongitudinal slot for its entire length so that it canbe axially andlongitudinally positioned within the coaxial line section 40 by anysuitable means as represented by the rod 64. The embodiment illustratedin FIG. 6 may be used to delay one signal relative to another, alsocontrollable signal, hereinafter referred to as the reference signal.The reference signal is transmitted through one section of line 40 andthe other signal is transmitted through the other section of line 40.By-longitudinally positioning tubular ferrite dielectric 62, the delayin one section of line is increased by a certain amount and the delay inthe other section is decreased by the same amount, and vice versa. Thenet result is that the maxi- ,mum difference in delay is doubled, yetthe space occupie'd by the line of FIG. 6 is only slightly larger thanthatoccupied by-the single delay line illustrated in FIGS.

land 2.

In-the embodiments described above the ferrite dielectric is shown ascomposed of a solid ferrite dielectric material. From an electricalviewpoint, the ferrite body may comprise alternating slices of ferritematerial and ceramic having a high dielectric constant. If theslices aremade so thin that their electrical length is smaller than about of thewavelength of the ferrite at the highest operating frequency, thelinestill acts as a uniform line. The effective permeability and theeffective dielectric constant in this arrangement are simply theweighted averages of the respective values of the materials. Withthisarrangement the average dielectric constant with the stacked slices canbe raised to any desired value.

While there has been described what is at present considered to be thepreferred embodiment of this invention, it will be obvious to thoseskilled in the art that various changes and modifications may be madetherein without departing from the invention, and it is therefore aimedin the appended claims to cover all such changes and modifications asfall within the true spirit and scope of the invention.

What is claimed is:

1. A continuously variable, low-loss delay line comprising a pair ofconductors, a ferrite dielectric intermediate said conductors, saidferrite dielectric being adapted to be slidably positioned with respectto said conductors such that when in one extreme position the ferritedielectric and saidconductors are coextensive, and when in the otherextreme position, the ferrite dielectric is completely removed from thespacing between said conductors and replaced by an-air dielectric, theeffective relative permeability a and the effective relative dielectrica being equal when the air spacing between said conductors is replacedby said ferrite dielectric.

2. The delay line in accordance with claim 1 wherein said conductorscomprise a strip-line.

3. A continuously variable, low-loss delay line comprising atransmission line section for propagating an RF signal including aninner conductor and an outer conductor coaxial therewith, a tubularferrite dielectric adapted to be axially positioned intermediate saidconductors such that when in one extreme position the ferrite dielectricand said conductor-s are coextensive, and when in the other extremeposition, the ferrite dielectric is completely removed from the spacingbetween said conductors and replaced by an air dielectric, said tubularferrite including a longitudinal slot, the width of said slot being suchthat the characteristic impedance of said transmission line sectionremains constant regardless of the relative axial position of thetubular ferrite dielectric between said inner and outer conductors.

4. The delay line in accordance with claim 3 wherein the tubular ferritedielectric is spaced from both said inner and outer conductors whenintermediate said conductors.

5. The delay line in accordance with claim 3 wherein the tubulardielectric is spaced from both said inner and outer conductorsintermediate said conductors.

6. The delay line in accordance with claim 5 wherein said outerconductor is terminated at one end by a plate axially spaced from saidinner conductor.

7. A continuously variable, low-loss delay line comprising atransmission line section including an inner conductor and an outerconductor coaxial therewith, respective RF input and output radialconductors afiixed to the respective ends of said inner conductor, saidradial conductors being linearly aligned and extending through saidouter conductor and insulated therefrom, a tubular ferrite dielectrichaving a longitudinal slot and adapted to be axially positionedintermediate said inner and outer conductors, said slot being alignedwith said radial conductors, the width of said slot being such that thecharacteristic impedance of said transmission line section remainsconstant when any portion of the air spacing between said inner andouter conductors is replaced by said ferrite dielectric.

8. The delay line in accordance with claim 7 wherein said outerconductor is terminated at the output radial conductor end by a plateaxially spaced from said inner conductor,

9. A continuously variable, low-loss delay line comprising atransmission line section including an inner conductor and an outerconductor coaxial therewith, respective RF input and output radialconductors aflixed to the respective ends of said inner conductor, saidradial conductors being linearly aligned and extending through saidouter conductor and insulated therefrom, a tubular ferrite dielectrichaving a longitudinal slot and adapted to be axially positionedintermediate said inner and outer conductors, said slots being alignedwith said radial conductors and bounded by radially disposed surfacesincluding an angle 6 therebetween, said angle 8 having a value such thatthe characteristic impedance of said transmission line section remainsconstant when any portion of the air spacing between said inner andouter conductors is replaced by said ferrite dielectric.

10. A continuously variable, low-loss delay line comprising atransmission line section for propagating an RF signal and including aninner conductor and an outer conductor coaxial therewith, a tubularferrite dielectric spaced from both said inner and outer conductors andadapted to be axially positioned intermediate said conductors such thatwhen in one extreme position the ferrite dielectric and said conductorsare coextensive, and when in the other extreme position the ferritedielectric is completely removed from the spacing between saidconductors and replaced by an air dielectric, the effective relativepermeability am and the effective relative dielectric a of said linebeing equal when the air spacing between said Conductors is replaced bysaid ferrite dielectric.

11. The delay line in accordance with claim 10 wherein the ferritedielectric comprises alternating pieces of a ferrite material and aceramic having a high dielectric constant.

References Cited by the Examiner UNITED STATES PATENTS 2,454,530 11/1948Tiley 333-81 3,092,793 6/1963 Augustine et a1. 333-31 3,145,353 10/1964Bleackley 33381 HERMAN KARL SAALBACH, Primary Examiner.

1. A CONTINUOUSLY VARIABLE, LOW-LOSS DELAY LINE COMPRISING A PAIR OFCONDUCTORS, A FERRITE DIELECTRIC INTERMEDIATE SAID CONDUCTORS, SAIDFERRITE DIELECTRIC BEING ADAPTED TO BE SLIDABLY POSITIONED WITH RESPECTTO SAID CONDUCTORS SUCH THAT WHEN IN ONE EXTREME POSITION THE FERRITEDIELECTRIC AND SAID CONDUCTORS ARE COEXTENSIVE, AND WHEN IN THE OTHEREXTREME POSITION, THE FERRITE DIELECTRIC IS COMPLETELY REMOVED FROM THESPACING BETWEEN SAID CONDUCTORS AND REPLACED BY AN AIR DIELECTRIC, THEEFFECTIVE RELATIVE PERMEABILITY UEFF AND THE EFFECTIVE RELATIVEDIELECTRIC EEFF BEING EQUAL WHEN THE AIR SPACING BETWEEN SAID CONDUCTORSIS REPLACED BY SAID FERRITE DIELECTRIC.